This invention relates to apparatus to provide exhaust ventilation by drawing air from one or more worksites in a large vessel.
While this invention has particular application to a continuous pulp digester of the KAMYR type which can be several hundred feet high, the term vessel is used herein to also include similar structures enclosing a confined space such as precipitators, storage tanks and refining columns. These types of large vessels require temporary ventilation when they are shut down periodically for maintenance and/or cleaning. Ventilation can be of the vessel as a whole which is referred to as general ventilation, or of specific areas or worksites which is referred to as local ventilation. In the past, only general ventilation has normally been provided to these types of vessels by mounting exhaust fans at an opening at the top end and/or blowers at an opening at the bottom end. An example of general ventilation apparatus mounted in a continuous pulp digester is shown in the applicants' Canadian Patent Application Serial Number 2,067,326 entitled "Digester Ventilation and Utility Module" which was published Oct. 28, 1993. While general ventilation is satisfactory for some situations, it is entirely inadequate if work, particularly welding is being done simultaneously at different elevations in the vessel. It will be appreciated that under such circumstances, the air in the vessel quickly becomes polluted with gases, fumes, smoke and particles which are very unpleasant and unhealthy. At present, using general ventilation to adequately clean the atmosphere requires an air velocity sufficient to suspend metal particles, as a result of grinding and arc air gouging, thereby causing numerous eye injuries. On the other hand, U.S. Pat. No. 1,046,884 to Spencer which issued Dec. 10, 1912 shows the concept similar to a central vacuum system of having a single air mover to which a number of different intake heads are connected. While this is also satisfactory for some situations, it has the disadvantages for a confined space in a high vessel that the single air mover must be extremely powerful and the exhaust duct must be very large. Thus, it is relatively costly and difficult to install, particularly in a crowded vessel where space to work is very limited.